Conventional electrostatic transducers, such as those used in certain loudspeakers, employ air gaps between conductive plates. The gap size in such transducers is reasonably large: typically in the range of 0.3 to 3 mm. Such transducers can be made using very thin film conductive sheets, for example a vacuum, deposited layer of aluminium on a very thin polymeric substrate such as polyester film. It is a fairly straightforward matter to design such transducers to provide a good impedance match to air. That is, the mechanical impedance associated with oscillations of the mass of the conductive sheets, and the mechanical impedance associated with the restorative force holding the sheets in place, is low compared to the mechanical impedance of air, so that most of the mechanical power of the transducer is conducted away as sound. Furthermore, such transducers can lend themselves to reasonably efficient conversion of electrical power to mechanical power, provided that the magnitude of the oscillation of the conductive sheets is a reasonable fraction of the size of the gap, which is often the case.
A fundamental difficulty with prior art electrostatic transducers like those described above is that they are incapable of providing large electrostatic pressures. That is, the quantity of force per unit area cannot be very large. This is because the electrostatic force per unit area in such transducers is given by the formula: EQU F/A=P=1/2 .sub.0 E.sup.2 ( 1)
where .sub.0 is 8.85.times.10.sup.-12 farads per metre and E is the magnitude of the electric field in the gap between the two conductive surfaces. For macroscopic gaps of the type described above, there is a maximum value of E, essentially independent of gap size, which is approximately 10.sup.6 volts per metre for room pressure air. This corresponds to a maximum electrostatic pressure of approximately 4.4 N/m.sup.2 or 0.00066 PSI. There are many applications where such forces are too low.
Recently, as described in U.S. Pat. No. 4,885,783 (Whitehead et al), practical methods have been devised for constructing electrostatic transducers employing very small air gaps (i.e. less than 10 micrometres) in which the breakdown field of air is substantially higher, due to the inhibition of avalanche breakdown from free ions, which is in turn due to the proximity of solid surfaces relative to the mean free path of a free ion in the gas. In such transducers a uniformly structured surface of elastomeric material creates air pockets of the desired size against a metal plate. Because the material is elastomeric it allows a reasonable amount of flexibility in the separation of the transducer's plates.
Such transducers are able to achieve pressures in the range of 100 to 1,000 times greater than conventional electrostatic transducers, and they are practical for certain applications where it is desirable to couple vibrational energy into media having an intermediate level of mechanical impedance, such as water, sand, soil, etc. However, the mechanical impedance of such transducers is too high to efficiently produce mechanical vibration in air. That is, most of the mechanical power of the transducer is associated with overcoming internal impedance, rather than in radiating mechanical energy into air. The transducer's higher impedance is partially due to the stiffness of the surface of the elastomeric material, but is predominately due to the difficulty of compressing the very shallow air pockets in the structure. Although air is compressible, the stiffness of an air gap is inversely proportional to the gap's thickness. For the small gap thicknesses characterizing transducers exemplified by U.S. Pat. No. 4,885,783 the resultant high stiffness becomes a significant problem. A further disadvantage of such devices is that it is often difficult to concentrate most of the electrostatic field energy in the active air gap, since generally speaking the elastomeric material is substantially thicker than the air gaps (pockets) in the surface of the material.
The present invention provides a transducer which can concentrate most of its electrostatic field energy in the active region of air gaps. The air gaps are small enough to achieve very high breakdown fields, and the structure is designed to reduce the high mechanical stiffness encountered in compressing narrow air gaps.
One specific requirement for producing acoustic excitation in air is in the field of aerodynamic flow. There is evidence that the characteristics of air flow over a rigid surface such as an airplane wing can be substantially modified by acoustic excitation. It would be highly desirable to employ a thin sheet transducer which could be easily applied to the surface of a structure such as a wing, to efficiently cause acoustic excitation of the air flowing over the surface. The prior art discloses no transducers capable of producing displacements of order several microns at frequencies of order 10 kilohertz, with efficiencies of order 1%. Such characteristics are highly desirable. It is a further object of this invention to fulfil this gap in the prior art.